The present invention relates to a cooling blade of a gas turbine.
Waste heat from a high-temperature gas turbine is recovered and utilized for a boiler of a steam turbine in some combined plants.
In one such combined plant, a cooling blade of the high-temperature gas turbine is cooled by internal cooling only. A refrigerant, having absorbed heat from the gas turbine by cooling it, is entirely recovered outside the gas turbine, and is fed into the steam turbine. By doing this, the performance of the whole plant is improved.
The cooling blade of the gas turbine has a recovery-type cooling structure. Thus, pneumatic power loss attributable to film-cooling that is applied to the high-temperature gas turbine can be reduced, and the turbine efficiency of the gas turbine can be improved.
Referring now to FIGS. 1A and 1B, a conventional turbine blade of the recovery type will be described. In the description to follow, a stationary blade is given as an example of the gas turbine cooling blade. FIG. 1A is a vertical sectional view of the cooling blade, and FIG. 1B is a sectional view taken along line A--A of FIG. 1A.
A cooling blade 1 is supplied with a refrigerant 6 from a waste heat boiler or the like outside a casing (not shown) through supply pipes 4 that penetrate an outer shroud 3. The refrigerant 6 fed into the blade 1 absorbs heat as it flows through a cooling passage 2 defined by a rib 7 toward a rear-side flow. The refrigerant 6, having thus absorbed heat, is guided through a recovery pipe to the outside of the gas turbine system. Thereafter, the refrigerant is delivered to a steam turbine in a combined plant, whereupon heat is recovered from it.
FIG. 2A shows a heat transfer rate distribution on the outer surface of the conventional turbine blade described above. Referring to FIG. 2A, a reference point O is settled on a front edge 8 (FIG. 1B) of the blade, and various points are settled on the outside and inside of the blade, corresponding individually to the ratios between X-direction distances from the reference point O, along the outside and inside of the blade, and a distance X.sub.max from the point O to the rear end edge of the blade. The curve of FIG. 2A represent a series heat transfer rates obtained at the individual points.
As seen from FIG. 2A, the heat transfer rate of the front edge 8 is relatively high, and the heat transfer area is narrower on the cooling side than on the gas side. Thus, the front edge 8 of the blade can be regarded as one of the most reluctant parts to be cooled.
Generally, in a cooling blade of this type, the refrigerant 6 is run through the cooling passage 2 therein, so that the passage 2 can be cooled at an average heat transfer rate throughout the area. Actually, however, high-temperature portions are formed in the cooling blade, as mentioned before. This implies that high-temperature portions will inevitably develop in part of the cooling blade if only the front edge of the blade, which displays relatively high heat transfer rates outside, is intensively cooled with use of a conventional serpentine passage.